1. Field of the Invention
The invention relates to a low-power DC-DC converter, and more particularly to a switched-capacitor DC-DC converter using one or two MOSFETS and not requiring inductive isolation.
2. Related Art
Challenges in designing high-efficiency auxiliary bias supplies
Auxiliary, low power, bias supplies are a necessity in all types of systems to provide local power for system controllers. In low input voltage systems, linear regulators and Zener/resistor combinations have been traditionally used for power conversion and, to this date, play a major role. High input voltages have always posed a major problem for linear regulators due to excessive losses generated by the linear regulators, rendering them undesirable.
In view of the growing need for local power, with the addition of safety, supervisory and regulatory concerns, designers have been forced to use more efficient methods to generate local power from high voltage sources.
Step down, inductor/transformer based, switching regulators offer a substantially more efficient conversion process over a higher/wider range of input voltages. Buck regulators have been used quite successfully in medium voltage applications, National Semiconductor""s xe2x80x9cSIMPLE SWITCHER(copyright) being a good example. One drawback of switching regulators is their tendency to generate EMI noise, which needs to be reduced by using proper input EMI filters, at an added cost. At even higher input voltages (such as off-line applications or systems with a PFC input stage), the buck regulator requires a fairly large inductor (10xcx9c20 mH) to limit the peak input current and reduce EMI, which can render it quite costly; conversely, the switching frequency can be increased to reduce the inductor requirement, at the expense of greater losses.
The fly-back converter is a good choice for high voltage applications, but by definition, requires an isolation transformer/inductor which not only adds to the overall system cost but is not always desired or required for local auxiliary supplies. Power Integration""s TINYSWITCH(copyright) and Maxim""s MAX5014(copyright) are good examples. Other proprietary controllers like the UC3889 attempt to address this issue by eliminating the transformer at the expense of an additional (and fairly large) inductor. Fly-back converters do not offer any substantial improvements with respect to EMI over buck converters.
Other solutions for auxiliary supplies in switching converters use an extra winding off the main transformer followed by a low pass filter and a Zener diode. Although this scheme is adequate for low power, and is widely used to power single controllers on the primary side, the requirement for an additional winding might not always be practical, or cost effective.
A low power auxiliary supply controller should provide as many of the following features as possible:
Low cost to allow replication within the system
Low power consumption for good efficiency. At low output power, controller losses will significantly impact overall efficiency.
Low EMI
Fairly good line/load regulation and no-load/short circuit protection
Ease of design-in.
Switched capacitor converters offer several advantages including low cost and inductor-less operation. It would be desirable to provide a switched capacitor converter that addresses the requirements for a low cost auxiliary supply controller.
Switched capacitor DC-DC converter basics
Switched capacitor (charge pump) DC-DC converters have been extensively used for positive to negative conversion of dc voltages (ICL766X , LTC660 . . . etc) as well as voltage doubling and buck/boost, integrated power converters. High efficiency switched capacitor converters traditionally have been limited to low input voltages (battery voltage conversion) and require high frequencies of operation. (LTC1911, LTC3250). At high input voltages, switched capacitor converters suffer from the following drawbacks:
High input peak currents at low operating frequencies.
Low efficiency at low operating frequencies. ( less than 1 MHz)
Low efficiency if input to output voltage differential is high.
A basic switched capacitor converter and its equivalent circuit are shown in FIG. 1. Switch SW1 is thrown from A to B at a rate determined by frequency f, thereby charging C1 to the input voltage Vin and discharging it into the load capacitance C2. In FIG. 1:
Req=1/Cl*freq
I=(Vinxe2x88x92Vout)/Req
Eloss=C1(Vin2xe2x88x92Vout2)/2
Ipeak=(Vinxe2x88x92Vout)/Rsw, where Rsw is the ON resistance of the switch
This conversion process will yield high efficiencies if input to output differential is small (Vin≈Vout). Also, high peak currents are avoided if input to output differential is small, contributing to reduced EMI.
A more efficient method is to use series charge and parallel discharge (equivalent circuits are shown FIG. 2). This method has the advantage of lower input peak current due to lower voltage differential at charge time (and also lower EMI) and is used for multi-stage integrated on-chip converters.
For high voltage discrete circuitry, it is desirable to reduce the number of external switches. FIG. 3 shows such an implementation. In a charge cycle, capacitors CF and CL are charged via switches SW1 and SW3, and switches SW2 and SW4 are OFF. In a discharge cycle, switches SW1 and SW3 are OFF, and capacitors CF and CL discharge to ground through switches SW2 and SW4.
A switched capacitor converter is presented as a low cost alternative to auxiliary supplies using buck or fly-back topologies in applications not requiring isolation. The system can operate over a wide range of input voltages up to 600 Vdc by proper selection of external components. Both fixed and variable output voltage versions are possible. The controller is designed to reduce EMI and provide an efficiency in the range of 70%.
According to a first aspect of the invention, a switched-capacitor converter may comprise a supply voltage input for receiving a supply voltage; an output circuit comprising a load resistance and a load capacitance connected in parallel; a diode circuit comprising a first diode and a second diode connected in series at a diode junction point, said diode circuit being connected in parallel with said output circuit; first and second semiconductor switches connected in series at a switch junction point; said semiconductor switches being connected between said supply voltage input and said output circuit; and a flying capacitance connected between said switch junction point and said diode junction point; wherein said load capacitance is charged via said flying capacitance and said second diode when said first switch is ON and said second switch is OFF, and said load capacitance is discharged via said first diode and said flying capacitance when said first switch is OFF and said second switch is ON. Each said semiconductor switch may include a p-channel device or an n-channel device. A current limiting component may be provided in series with at least one of said first diode, said flying capacitor, said second switch and said output circuit. The current limiting component may be an inductance and may be disposed between said first diode and said diode junction point.
According to a second aspect of the invention, a control IC for controlling said first and second semiconductor switches may comprise a high side well powered by said input supply voltage and including a first driver circuit connected for driving said first semiconductor switch; a floating well powered by said flying capacitor and including a second driver circuit connected for driving said second semiconductor switch; and a control circuit powered by an output voltage across said output circuit. Advantageously, at a start-up time, said control circuit charges said load capacitance to a predetermined initial voltage. It may control said first semiconductor switch to charge said flying capacitance and thereby charge said load capacitance to said predetermined initial voltage. Also advantageously, said control circuit delivers a variable amount of charge to said load capacitance per unit time. Said control circuit may increase a discharge frequency in response to an increase in load power demand, and maintain a predetermined fixed discharge time. Also, when said discharge frequency reaches a predetermined maximum, the control circuit maintains said maximum frequency and increases the discharge time in response to an increase in load power demand.
According to a third aspect of the invention, a switched-capacitor converter may comprise a supply voltage input for receiving a supply voltage; an output circuit comprising a load resistance and a load capacitance connected in parallel; a diode circuit comprising a first diode and a second diode connected in series at a diode junction point, said diode circuit being connected in parallel with said output circuit; first and second semiconductor switches connected in series at a switch junction point; said semiconductor switches being connected between said supply voltage input and said output circuit; and a flying capacitance connected across said second semiconductor switch and to said diode junction point; wherein said load capacitance is charged via said flying capacitance and said second diode when said first switch is ON and said second switch is OFF, and said load capacitance is discharged via said first diode and said flying capacitance when said first switch is OFF and said second switch is ON. Said first and second semiconductor switches may be interconnected by a third diode, said flying capacitor being connected to said diode connection point of said first and second diodes, and to a connection point between said third diode and said second semiconductor switch. Also, said first and second semiconductor switches may be connected to each other directly.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.